$-\dfrac{3}{4} - \dfrac{3}{5} = {?}$
Solution: $ = - {\dfrac{3 \times 5}{4 \times 5}} - {\dfrac{3 \times 4}{5 \times 4}} $ $ = - {\dfrac{15}{20}} - {\dfrac{12}{20}} $ $ = - \dfrac{{15} - {12}}{20} $ $ = -\dfrac{27}{20}$